Aircraft fuel measuring systems are required to measure mass which is a product of density and volume. Prior art mechanisms for monitoring fuel density have included vibrating spools, hydrometer floats and inference based systems using dielectric constant or speed of sound.
U.S. Pat. No. 4,232,544 which issued to James W. Stansfeld and is assigned to Solartron Electronic Group, Limited discloses apparatus for sensing the density of a fluid. Similar structures are disclosed in U.S. Pat. Nos. 4,354,377 to Stansfeld and 4,466,272 to Stansfeld. Each of these prior art patents discloses an elongated tube and an electrical transducer for exciting the elongated tube at its natural frequency of vibration. Changes of the density of a fluid contacting the tube modify the frequency of vibration. By correlating this change in frequency with a change in density, it is possible for such structures to determine the density of a fluid. Systems embodying a vibrating tube sensing device such as those disclosed in these patents can produce accurate results, but are expensive. Additionally, the size of such systems can make them inappropriate for certain applications.
Another category of prior art densitometers are commercially available from Simmonds Precision Aircraft Systems, Division of B. F. Goodrich Aerospace, assignee of the present invention. One such system uses an ultrasonic gauging technique which senses the speed at which sound passes through the fluid and the temperature of the fluid to determine a density as disclosed in U.S. Pat. No. 4,815,323 to Ellinger et al.
It is also known in the prior art to determine fuel density by measuring the dielectric constant of the fuel using a capacitance measuring technique. These systems infer a density based upon the measured dielectric constant of the fuel. While these systems are less costly to implement than the densitometer using a vibrating tube, they are also somewhat less accurate.